Inductively coupled plasma mass spectroscopy (ICP-MS) apparatuses have a dynamic range of measured signals as wide as nine digits, and the signal measurement is usually performed by a plurality of methods in accordance with an intensity of the signal. Therefore, it is necessary to associate a plurality of types of measured values determined by the individual methods by calibration. Typically, there are adopted two methods including a pulse count method and an analog current method. The calibration in this case is performed by measuring in a calibration range that is an overlapping signal range in which the measured signals determined in the two methods are both effective and by determining a ratio between measured signal levels of both methods. This ratio is often referred to as the pulse-to-analog (P/A) coefficient. Typically, it is desirable to perform the calibration individually for each element to be measured because the ratio of signal levels is different depending on the element to be measured.
In general, in order to obtain an effective P/A coefficient of a certain mass number, it is necessary to calculate the P/A coefficient between the analog current value and the pulse count value determined in a P/A coefficient calibration range (hereinafter, referred to as calibration range) that is an overlapping signal intensity range in which both the analog current value and the pulse count value measured for the mass number are effective. In addition, the signal intensity with respect to the sample density is different depending on the element. Therefore, it is necessary in the known method to prepare samples for P/A coefficient calibration having different densities for each element so that a signal in the calibration range can be determined for each element, and to perform measurement for determining the P/A coefficient. As such, determination of the P/A coefficient by this known ICP-MS can be labor-intensive.
In another known system, a measured value of the element in the sample to be measured is used for determining the P/A coefficient, and the density of the element in the sample to be measured is uncertain. For this reason, if the sample to be measured contains a low density element such that only a measured value in the pulse only region can be determined even if the transmission ratio of an ion lens is maximized for each element, the measured value of the element can be outside the calibration range by adjustment of the transmission ratio of the ion lens. Therefore, the P/A coefficient cannot be determined for such element.
Further, in the known system, the voltage to be applied to the ion lens 30 is changed step by step so that the measured signal can be determined over a wide range for determining the measured value of each element securely in the calibration range for every desired element in the sample to be measured. This method needs a lot of time for measurement to determine the P/A coefficient because the measurement is performed at many voltage points.
In addition, in the known system special hardware is needed such as a comparator for comparing the measured analog signal voltage with some predetermined voltages so as to discriminate which signal region the measured signal belongs to. Therefore, it is not easy to perform the known method in connection with a known ICP-MS.
Moreover, in the known ICP-MS described above an element of the sample may have a mass number having a P/A coefficient that is not determined by the ICP-MS 1, or an element of the sample may have a signal intensity that cannot measured in the calibration range because the density in the sample for P/A calibration is too low.
In this case, it is regarded that the P/A coefficient of the element exists only in the mass number of the element, and an linear interpolation approach is adopted in which known P/A coefficients of two different mass numbers having a relationship that a mass number having an undetermined P/A coefficient exists between them are connected by a straight line and a value on the straight line of the mass number for which the P/A coefficient is to be determined is regarded as an estimated value of the P/A coefficient of the mass number.
FIG. 11 is a graph showing an example of estimation of the P/A coefficient by the known linear interpolation based on a dependence of the P/A coefficient on the mass number. The horizontal axis (x axis) represents the mass number, and the vertical axis (y axis) represents the P/A coefficient. Five points indicated by squares in the graph are obtained by plotting the P/A coefficients that are determined in advance in the ICP-MS 1 with respect to the mass numbers.
Each of the four straight lines illustrated by the dot lines in FIG. 11 connects two points having neighboring mass numbers among the five points. For instance, the straight line part L in FIG. 11 connects the P/A coefficients corresponding to the mass numbers of 69 amu and 238 amu, respectively. The P/A coefficient corresponding to the mass number of 137 of the element Ba between the two mass numbers is estimated as a y coordinate value when the x coordinate value is 137 in the linear equation representing the straight line L.
However, the P/A coefficient depends not just on the mass number, and hence an error of the estimated value is apt to increase relatively when the P/A coefficient is estimated by the linear interpolation on the basis of only the dependence on the mass number.
What is needed therefore is a method and apparatus for estimating a P/A coefficient that overcomes at least the drawbacks of known methods described above.